With increasing communication data and communication speed, active research and development for an advanced information security technology is under way. For an information security technology, a quantum info-communication via a single-photon in a laser light is drawing attention. Quantum info-communication is a communication method in which using an optical fiber, made of a conventional optical fiber for communication a part of which is thinned (also referred to as “tapered optical fiber”), a single atom or a quantum dot is arranged in the tapered optical fiber to be excited by a laser, and the generated photon is taken into a fiber propagation mode.
As a technique for thinning an optical fiber, proposed are techniques described in Patent Documents 1 to 4 below related to an optical fiber coupler which draws and fuses a plurality of optical fibers. For example, Patent Document 1 proposes a technique relating to a manufacturing method of an optical fiber coupler. The manufacturing method of an optical fiber coupler includes: an optical fiber setting step in which at least three optical fibers, from which a jacket is removed, are inserted into a microheater from an opening thereof and arranged parallel to each other in a direction orthogonal or nearly orthogonal to the direction of the opening; and an optical fiber heating and fusing step in which the microheater is reciprocated within a prescribed width dimension in the axis direction of the three optical fibers to heat and fuse the optical fibers, and the optical fibers are drawn in the axis direction. In this technique, optical fibers adjacent to each other in a state, in which at least three optical fibers are arranged in parallel, are uniformly heated to be subjected to a fusing treatment to attain a state in which optical fibers do not have irregularity, thereby obtaining a highly accurate optical fiber coupler.
Patent Document 2 proposes a technique regarding a manufacturing system of an optical fiber coupler. In Patent Document 2, a microheater mounted on a heater support heats and fuses in a prescribed width dimension of cores of two optical fibers from which a jacket is removed and which are arranged adjacent to each other. The manufacturing system of the optical fiber coupler is characterized in that the above-mentioned microheater is mounted on a heater support via a reciprocating shifter, in order to adjust the length of a heated and fused portion to any length, and to reciprocate the microheater in the axis direction of two optical fibers. By this technique, a heated and fused portion of an optical fiber coupler which is heated and fused by a microheater can be set to have a large width dimension and any width dimension, thereby obtaining a sufficient intensity and setting any branch state of a light.
Patent Document 3 proposes a technique regarding a manufacturing method of a wide-band optical fiber coupler formed by heating one optical fiber to be subjected to a predrawing process, and fusing and drawing the optical fiber with unprocessed optical fiber by heating. The manufacturing method of a wide-band optical fiber coupler includes: allowing a heating system to move along an optical fiber to heat the optical fiber in a predrawing process; starting drawing of the optical fiber when the heating system reaches the end of the region to be heated; and predrawing the optical fiber to a prescribed length. By this technique, a plurality of optical fibers is fused without a gap at fusing rate of 100%, thereby effectively manufacturing optical fiber couplers at high yield.
Patent Document 4 proposes a technique regarding a manufacturing method of an optical fiber coupler in which part of a plurality of optical fibers are heated, fused, and drawn to form a fused and drawn portion, and a light is branched and coupled by the fused and drawn portion. The manufacturing method of an optical fiber coupler performs to fuse and draw a plurality of optical fibers while measuring the tension exerted on the plurality of optical fibers and while controlling the drawing speed of the optical fibers or the drawing speed and the heating condition of the optical fibers in order to make the tension close to zero. By this technique, deterioration of the optical properties (in particular, the polarization dependence) of the optical fiber coupler due to generation of a tension can be prevented, whereby an optical fiber coupler having a high performance optical fiber can be produced.